The present invention relates to a tuning circuit for a superheterodyne radio receiver, and more particularly to such a tuning circuit with automatic frequency control (AFC) of the local oscillator.
A tuning circuit for a superheterodyne receiver with automatic frequency control is described in applicant's above-identified parent application Ser. No. 608,350, the subject matter of which is hereby incorporated by reference. According to the system described in that application the local oscillator frequency is automatically regulated, by means of a control voltage, to the value of the rated frequency associated with the receiving channel, i.e. the received frequency, if there is a deviation of the oscillator frequency from the rated frequency. The control voltage is generated on the basis of the deviation of the oscillator frequency from the rated frequency.
The invention in the above-identified application resides in the provision of a counter which counts the local oscillator oscillations and from whose counter state the control voltage is derived. The frequency steps counted by the counter are here smaller than the frequency steps between the rated frequencies to which certain counter states are associated. This eliminates a significant drawback of known AFC circuits in which, if there are two stations close together on the frequency scale, the receiver is inevitably tuned to the station having the strongest receiving field.